Enhanced Interior Gateway Routing Protocol (EIGRP)

Author: Dheeraj (Raj) Tolani, Global Knowledge Instructor

Abstract

Enhanced Interior Gateway Routing Protocol (EIGRP) is a proprietary Cisco advanced distance-vector classless routing protocol. By "advanced," we are saying that it is better than the other distance vector protocols, like RIP (Routing Information Protocol), for example. "Classless" means that it will be advertising the subnet mask information in its updates. This white paper will introduce you to EIGRP and discuss some of its new features.

Sample

Introduction

Every routing protocol has some pros and cons and its own metric considerations. In this white paper, we will discuss Enhanced Interior Gateway Routing Protocol (EIGRP), a Cisco-proprietary routing protocol.

EIGRP is an advanced distance-vector classless routing protocol. By using the word advanced, we are basically saying that it is better than the other distance vector protocols, like RIP (Routing Information Protocol) for example. By classless, we mean that it will advertise the subnet mask information in its updates.

Before going to EIGRP, Cisco had another protocol called IGRP (Interior Gateway Routing Protocol). Coming from an older protocol, EIGRP still has some of the IGRP limitations, like automatic summarization at network boundary. This limitation can be easily fixed, however, with no auto-summary command under router configuration mode. These limitations and fixes are reviewed in detail in Global Knoweldge's BSCI class.

Features of EIGRP

• Since it is advertising its subnet mask information, you could use different subnet masks at different places in your network, based on your host requirements or other business reasons. This means there is a VLSM (Variable Length Subnet Mask) support in EIGRP.
• EIGRP allows you implement unequal load sharing alongside equal load sharing, if you have parallel links between sites.
• EIGRP allows you to specify the different times of the day when different passwords will be effective. (I hope your time servers are keeping things in sync so you can take advantage of this great feature.)
• EIGRP uses IP protocol # 88, so if you are the firewall person or the ACL person, you want to ensure that the protocol number 88 is allowed. Remember, EIGRP rides on IP, not on TCP or UDP like some other protocols do (RIP, for example, rides on UDP port # 520).
• EIGRP allows you to use MD5 authentication between peers. This could be very powerful if you are afraid that someone could be sniffing the network and stealing your routes. With EIGRP authentication, you can validate each update and ensure that you are talking to the correct EIGRP neighbor since they share the same password.

Autonomous Systems and Neighbors

In the early days, when running RIP, one of the issues was the lack of ability to group RIP routers under one administration unit. IGRP gave us the ability to have something called an Autonomous System Number. EIGRP built on that, and now we can group routers under a common administration unit called the Autonomous System.

You can assign any number in the range 1-65,535 to the routers. In order for them to incorporate routes they hear from other EIGRP-speaking routers, the Autonomous System number (AS#) must match. If the AS# doesn't match, then the routes will be discarded.

In order to start speaking EIGRP with other routers, we have to ensure that we are set up as neighbors. In addition to having the same AS#, we must also be on the same L3 network. By default, EIGRP uses the primary interface IP address as the source IP address, so it must be on the same broadcast domain (L3) as the other routers it is trying to talk to.

We said earlier that you can pick any AS number in the range of 1-65,535, but some of my clients use the same number that they use with their external ISP connections running BGP. EIGRP doesn't require this but, if it makes sense to you, then why not.

For metric calculations, EIGRP metric is a 32-bit number. These are big, ugly numbers that are based on a Ciscoproprietary Diffusing Update Algorithm (DUAL). By default, EIGRP uses "K-values" Bandwidth (BW) and Delay (DLY). If you want to, you can also factor in Reliability (REL) and Load, though these are not in use with "out of the box" EIGRP configurations. MTU is also exchanged but not used for metric calculations. So, in order for the EIGRP neighbor relationship to be formed exchanging routes, the K-values must match in the hello messages. Configuration In order to configure EIGRP you must be in the router configuration mode. Then, just as with Access Control Lists or OSPF network configurations, you can use the wildcard mask to specify the network/host addresses you want to start EIGRP. If you don't key in the wildcard mask entries, the system defaults to classful assumption.